Recent advances in our comprehension of mammalian immunological responses have led to the prevention of certain diseases in man through prophylactic vaccination and the control and treatment of diseases by the use of immunotherapeutics. The types of diseases which may be addressed through immunological intervention include those caused by infectious agents, cancers, allergies and autoimmune diseases. In these cases, most commonly, the premise of the medical treatment is the efficient delivery of antigens to appropriate immune recognition cells. For example, prophylactic vaccination has successfully eradicated smallpox worldwide through the administration of a live attenuated strain of the virus bearing all the antigens of the wild type virus. Similarly infections due to the Haemophilus influenzae serotype b bacterium have been significantly reduced in Western countries following the development of a vaccine based upon the polysaccharide antigen from the bacterial cell wall. Moreover, some cancers such as human melanoma respond to immunotherapy using autologous dendritic cells (DC) as a cellular adjuvant and defined peptides derived from the melanosomal protein gp100 as the source of tumour-specific antigen to generate a cell-mediated immune response.
Self-tolerance to autoantigen can be restored in the treatment of experimental autoimmune encephalomyelitis by injection of a specific neuroantigen that is the target of the destructive immune response. Hence specificity can be afforded by such treatment without the need for long-term immunosuppression.
For infectious diseases, the most rapid progress in disease control has occurred where antibody raised to the administered antigen is capable of neutralising the infectious agent or toxin secreted therefrom, whether this be mediated through IgM, IgG or IgA. Likewise, autoimmune diseases have been treated with antigens that can ameliorate the action of auto-antibodies. However, for the eradication of virus-infected cells, cancer cells and cells harbouring intracellular bacteria, cellular immune responses are also required. For example, intracellular viruses (e.g. retroviruses, oncornaviruses, orthomyxoviruses, paramyxoviruses, togaviruses, rhabdoviruses, arenaviruses, adenoviruses, herpesviruses, poxviruses, papovaviruses and rubella viruses) are able to replicate and spread to adjacent cells without becoming exposed to antibody. The importance of cell-mediated immunity is emphasised by the inability of children with primary T-cell deficiency to clear these viruses, whilst patients with immunoglobulin deficiency but intact cell-mediated immunity do not suffer this handicap. A small, but important, number of bacteria, fungi, protozoa and parasites survive and replicate inside host cells. These organisms include Mycobacteria (tuberculosis and leprosy), Legionella (Legionnaires Disease), Rickettsiae (Rocky Mountain spotted fever), Chlamydiae, Listeria monocytogenes, Brucella, Toxoplasma gondii, Leishmania, Trypanosoma, Candida albicans, Cryptococcus, Rhodotorula and Pneumocystis. By living inside cells, these organisms are inaccessible to circulating antibodies. Innate immune responses are also ineffective. The major immune defense against these organisms is cell-mediated immunity; involving both CD8+ cytolytic T Lymphocytes and CD4 helper T Lymphocytes.
The development of vaccines and immunotherapeutics capable of eliciting an effective and sustained cell-mediated immune response remains one of the greatest challenges in vaccinology. In particular the development of a safe and efficacious vaccine for the prevention and treatment of Human Immunodeficiency Virus (HIV) infection has been hindered by the inability of vaccine candidates to stimulate robust, durable and disease-relevant cellular immunity.
The host cell-mediated immune response responsible for eradicating intracellular pathogens or cancer cells is termed the Th1 response. This is characterised by the induction of cytotoxic T-lymphocytes (CTL) and T-helper lymphocytes (HTL) leading to the activation of immune effector mechanisms as well as immunostimulatory cytokines such as IFN-gamma and IL-2. The importance of Th1 responses in the control of viral infection has recently been shown by Lu et al. (Nature Medicine (2004)). This clinical study with chronically HIV-1 infected individuals demonstrated a positive correlation between the suppression of viral load and both the HIV-1-specific IL-2- or IFN-gamma-expressing CD4+ T cells and specific HIV-1 CD8+ effector cell responses. Current immunological strategies to improve the cellular immunity induced by vaccines and immunotherapeutics include the development of live attenuated versions of the pathogen and the use of live vectors to deliver appropriate antigens or DNA coding for such antigens. Such approaches are limited by safety considerations within an increasingly stringent regulatory environment. Furthermore, issues arising from the scalability of manufacturing processes and cost often limit the commercial viability of products of biological origin.
In this context, rationally defined synthetic vaccines based on the use of peptides have received considerable attention as potential candidates for the development of novel prophylactic vaccines and immunotherapeutics. T cell and B cell epitopes represent the only active part of an immunogen that are recognized by the adaptive immune system. Small peptides covering T or B cell epitope regions can be used as immunogens to induce an immune response that is ultimately cross-reactive with the native antigen from which the sequence was derived. Peptides are very attractive antigens as they are chemically well-defined, highly stable and can be designed to contain T and B cell epitopes. T cell epitopes, including CTL and T helper epitopes, can be selected on the basis of their ability to bind MHC molecules in such a way that broad population coverage can be achieved (The HLA Factsbook, Marsh, S., Academic Press. 2000). Moreover, the ability to select appropriate T and B cell epitopes enable the immune response to be directed to multiple conserved epitopes of pathogens which are characterised by high sequence variability (such as HIV, hepatitis C virus (HCV), and malaria).
In order to stimulate T lymphocyte responses, synthetic peptides contained in a vaccine or an immunotherapeutic product should preferably be internalized by antigen presenting cells and especially dendritic cells. Dendritic cells (DCs) play a crucial role in the initiation of primary T-cell mediated immune responses. These cells exist in two major stages of maturation associated with different functions. Immature dendritic cells (iDCs) are located in most tissues or in the circulation and are recruited into inflamed sites. They are highly specialised antigen-capturing cells, expressing large amounts of receptors involved in antigen uptake and phagocytosis. Following antigen capture and processing, iDCs move to local T-cell locations in the lymph nodes or spleen. During this process, DCs lose their antigen-capturing capacity turning into immunostimulatory mature Dcs (mDCs).
Dendritic cells are efficient presenting cells that initiate the host's immune response to peptide antigen associated with class I and class II MHC molecules. They are able to prime naïve CD4 and CD8 T-cells. According to current models of antigen processing and presentation pathways, exogeneous antigens are internalised into the endocytic compartments of antigen presenting cells where they are degraded into peptides, some of which bind to MHC class II molecules. The mature MHC class II/peptide complexes are then transported to the cell surface for presentation to CD4 T-lymphocytes. In contrast, endogenous antigen is degraded in the cytoplasm by the action of the proteosome before being transported into the cytoplasm where they bind to nascent MHC class 1 molecules. Stable MHC class 1 molecules complexed to peptides are then transported to the cell surface to stimulate CD8 CTL. Exogenous antigen may also be presented on MHC class molecules by professional APCs in a process called cross-presentation. Phagosomes containing extracellular antigen may fuse with reticulum endoplasmic and antigen may gain the machinery necessary to load peptide onto MHC class 1 molecules. It is well recognised, however, that free peptides are often poor immunogens on their own (Fields Virology, Volume 1, Third Edition, 1996).
To optimise the efficacy of peptide vaccines or therapeutics, various vaccine strategies have been developed to direct the antigens into the antigen-presenting cell in order to target the MHC class I pathway and to elicit cytotoxic T-lymphocyte (CTL) responses. As an example of a synthetic delivery system, fatty acyl chains have been covalently linked to peptides as a means of delivering an epitope into the MHC class I intracellular compartment in order to induce CTL activity. Such lipopeptides, for example with a monopalmitoyl chain linked to a peptide representing an epitope from HIV Env protein are described in the U.S. Pat. No. 5,871,746. Other technologies have been delivered that aim to deliver epitopes into the intracellular compartment and thereby induce CTLs. These include vectors such as Penetratin, TAT and its derivatives, DNA, viral vectors, virosomes and liposomes. However, these systems either elicit very weak CTL responses, have associated toxicity issues or are complicated and expensive to manufacture at the commercial scale.
There is therefore a recognised need for improved vectors to direct the intracellular delivery of antigens in the development of vaccines and drugs intended to elicit a cellular immune response. A vector in the context of immunotherapeutics or vaccines is any agent capable of transporting or directing an antigen to immune responsive cells in a host. Fluorinated surfactants have been shown to have lower critical micellar concentrations than their hydrogenated counterparts and thus self-organise into micelle structures at a lower concentration than the equivalent hydrocarbon molecule. This physicochemical property is related to the strong hydrophobic interactions and low Van der Waal's interactions associated with fluorinated chains which dramatically increase the tendency of fluorinated amphiphiles to self-assemble in water and to collect at interfaces. The formation of such macromolecular structures facilitates their endocytic uptake by cells, for example antigen-presenting cells (Reichel F. et al. J. Am. Chem. Soc. 1999, 121, 7989-7997). Furthermore haemolytic activity is strongly reduced and often suppressed when fluorinated chains are introduced into a surfactant (Riess, J. G.; Pace, S.; Zarif, L. Adv. Mater. 1991, 3, 249-251) thereby leading to a reduction in cellular toxicity.